When an encoding/decoding method is differently applied to an input signal where a speech and audio are combined depending on a characteristic of the input signal, a performance and a sound quality may be improved. For example, it may be efficient to apply a Code Excited Linear Prediction (CELP)-based encoder to a signal having a similar characteristic to a speech signal, and to apply a frequency conversion-based encoder to a signal identical to an audio signal.
A Unified Speech and Audio Coding (USAC) may be developed by applying the above-described concepts. The USAC may continuously receive an input signal and analyze a characteristic of the input signal at particular times. Then, the USAC may encode the input signal by applying different types of encoding apparatuses through switching depending on the characteristic of the input signal.
A signal artifact may be generated during signal switching in the USAC. Since the USAC encodes an input signal for each block, a blocking artifact may be generated when different types of encodings are applied. To overcome such a disadvantage, the USAC may perform an overlap-add operation by applying a window to blocks where different encodings are applied. However, additional bitstream information may be required due to the overlap, and when switching frequently occurs, an additional bitstream to remove blocking artifact may increase. When a bitstream increases, an encoding efficiency may be reduced.
In particular, the USAC may encode an audio characteristic signal using a Modified Discrete Cosine Transform (MDCT)-based encoding apparatus. An MDCT scheme may transform an input signal of a time domain into an input signal of a frequency domain, and perform an overlap-add operation among blocks. In an MDCT scheme, aliasing may be generated in a time domain, whereas a bit rate may not increase even when an overlap-add operation is performed.
In this instance, a 50% overlap-add operation is to be performed with a neighbor block to restore an input signal based on an MDCT scheme. That is, a current block to be outputted may be decoded depending on an output result of a previous block. However, when the previous block is not encoded using the USAC using an MDCT scheme, the current block, encoded using the MDCT scheme, may not be decoded through an overlap-add operation since MDCT information of the previous block may not be used. Accordingly, the USAC may additionally require the MDCT information of the previous block, when encoding a current block using an MDCT scheme after switching.
When switching frequently occurs, additional MDCT information for decoding may be increased in proportion to the number of switchings. In this instance, a bit rate may increase due to the additional MDCT information, and a coding efficiency may significantly decrease. Accordingly, a method that may remove blocking artifact and reduce the additional MDCT information during switching is required.